Recently I've had occasion to help several people trying to convert thread pitches between metric and inferial. In the process of documenting this for them, I noticed something that's worth remembering...
Basically, it goes like this: Whatever you have, metric or inferial, divide it into 25.4 to obtain the other.
A simple units analysis will prove this...
M = metric pitch measured in mm/th(read)
E = inferial pitch measured in tpi [th(read)/inch]
25.4 (mm/in) / M (mm/th) = (25.4 / M) (mm/in * th/mm) = 25.4 / M (th/in) = 25.4 / M tpi
25.4 (mm/in) / E (th/in) = (25.4 / E) (mm/in * in/th) = 25.4 / E (mm/th)
i.e., whatever you've got, E or M, divide it into 25.4 to get the other